Watch for the day-and-a-half old moon to appear low in the west shortly after sunset tonight. Binoculars will more clearly show the moon's complete outline. Direct sunlight illuminates the crescent. Sunlight reflected from Earth faintly lights the rest of the disk. Stellarium
Watch for the day-and-a-half old moon to appear low in the west shortly after sunset tonight. Binoculars will more clearly show the moon's complete outline. Direct sunlight illuminates the crescent. Sunlight reflected from Earth faintly lights the rest of the disk. Stellarium

Tonight the crescent moon comes back to the evening sky. You'll see it low in the west starting about 20-30 minutes after sunset. By tomorrow it will stand further from the sun and higher, making it even easier to spot. On both nights, earthshine will add an extra layer of enjoyment. Use binoculars to see this twice-reflected light best.

The moon orbits once every 27 days during which time its phase changes from crescent to full to crescent again as seen from the Earth. The sun is off to the right in this illustration. ESA
The moon orbits once every 27 days during which time its phase changes from crescent to full to crescent again as seen from the Earth. The sun is off to the right in this illustration. ESA

The moon orbits the Earth every 27 days, moving eastward (to the left in the northern hemisphere) at the at about 13 a day, roughly one horizontal, balled fist held at arm's length against the sky. That's also the equivalent ofone moon-diameter per hour. Its night-to-night motion is easy to see, but hour-to-hour not so much. For that you need a marker close by like a bright planet or star.

Fortunately, that happens during conjunctions when the moon pairs up with a celestial companion. If you check on the moon hourly during one of these trysts you'll see it slowly move toward or away from the star or planet. No bright conjunctions are on the docket tonight or tomorrow night, but if you're patient a wonderful opportunity lies ahead.

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On September 21, the last day of summer, the waxing crescent not only aligns with the modestly bright star Acrab (Beta Scorpii) in Scorpius, but from many locations it will temporarilyoccult or hide the star. This should be a very exciting event to observe with binoculars and small telescopes. I'll have more to say about it here later this weekend.

Our best theory on the moon's formation involves a giant impact about 4.5 million years ago. The debris released formed a ring around the early Earth which then coalesced into the moon. The putative planet smasher is named Theia. Citronade / CC BY-SA 4.0
Our best theory on the moon's formation involves a giant impact about 4.5 million years ago. The debris released formed a ring around the early Earth which then coalesced into the moon. The putative planet smasher is named Theia. Citronade / CC BY-SA 4.0

In the meantime, enjoy watching the moon work its way around the Earth while chewing on this thought. A titanic collision between a Mars-sized protoplanet and the Earth 4.4 - 4.45 billion years ago likely created the moon. The material first formed a ring around the Earth before coming together to mold the gray sphere that 4.5 billion years later puts a haunt in Halloween. The original energy of the impact and subsequent capture of the material by Earth's gravity continue to propel the moon around our planet at an average speed of 2,288 mph (3,683 km). That's very close to the speed of one of the fastest non-experimental, manned aircraft, the Lockheed SR-71 Blackbird, which could do 2,193 mph (3,530 kph).